conf: improve the way we format blkiotune and cputune
conf: improve the way we format blkiotune and cputune

Just refactor existing code to use a child buf instead of
check all element before format <blkiotune> and <cputune>.
This will avoid the more and more bigger element check during
we introduce new elements in <blkiotune> and <cputune> in the
future.

Signed-off-by: Luyao Huang <23d9981432efa88691fd2a71f5525bc2174f7664@redhat.com>

nertpinx/libvirt,fabianfreyer/libvirt,VenkatDatta/libvirt,nertpinx/libvirt,datto/libvirt,agx/libvirt,agx/libvirt,jardasgit/libvirt,taget/libvirt,rlaager/libvirt,olafhering/libvirt,rlaager/libvirt,andreabolognani/libvirt,olafhering/libvirt,jfehlig/libvirt,datto/libvirt,jfehlig/libvirt,libvirt/libvirt,datto/libvirt,agx/libvirt,jfehlig/libvirt,agx/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,olafhering/libvirt,jardasgit/libvirt,jardasgit/libvirt,crobinso/libvirt,zippy2/libvirt,jfehlig/libvirt,rlaager/libvirt,andreabolognani/libvirt,nertpinx/libvirt,eskultety/libvirt,andreabolognani/libvirt,agx/libvirt,zippy2/libvirt,taget/libvirt,eskultety/libvirt,crobinso/libvirt,VenkatDatta/libvirt,eskultety/libvirt,zippy2/libvirt,zippy2/libvirt,crobinso/libvirt,VenkatDatta/libvirt,libvirt/libvirt,crobinso/libvirt,taget/libvirt,fabianfreyer/libvirt,VenkatDatta/libvirt,nertpinx/libvirt,VenkatDatta/libvirt,taget/libvirt,fabianfreyer/libvirt,rlaager/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,jardasgit/libvirt,rlaager/libvirt,libvirt/libvirt,fabianfreyer/libvirt,datto/libvirt,libvirt/libvirt,eskultety/libvirt,jardasgit/libvirt,olafhering/libvirt,eskultety/libvirt,datto/libvirt,taget/libvirt